def convert(c, key, start = 'a'):
    a = ord(start)
    offset = ((ord(c) - a + key)%26)
    return chr(a + offset)
def encrypt(s, key):
    m = ""
    for c in s:
        if c.islower():
            m+= convert(c, key, 'a')
        elif c.isupper():
            m+= convert(c, key, 'A')
        else:
            m+= c
    return m
def decrypt(s, key):
    return encrypt(s, -key)
if __name__ == '__main__':
    key = 3
    cipher = "RPQLD TDOOLD HVW GLYLVD LQ SDUWHV WUHV"
    mes = decrypt(cipher,key)
    print(mes)